This project concerns the cellular and molecular mechanisms controlling blood cell differentiation and proliferation of pluripotent hemopoietic stem cells. We are studying the effects of erythropoietin, the primary inducer of red blood cell differentiation, and of "colony stimulating factor," a substance that may be granulopoietin, on granulocyte-macrophage colony formation, on erythroid colony formation and on induced hemoglobin synthesis by marrow cells in culture. The concepts under investigation relate to whether pluripotent stem cells are the direct loci of action of inducers of differentiation, or whether committed, unipotent cells are the site of action. These alternatives are being tested by experiments designed to determine whether there are competitive effects between the two inducers. We propose to extend these studies to the third line of a blood cell differentiation, megakaryopoiesis and, in addition, to initiate a study of whether colony stimulating factor is a true granulopoietin. We are also studying the long-term maintenance, in culture, of pluripotent stem cells and the fractionation of bone marrow into target cell populations for the inducers. The mechanisms by which the external signal, erythropoietin, acts on the cell to initiate nuclear transcription, the characterization of the cytoplasmic mediator protein, and the relationships between the receptors for the various inducers of differentiation are also under study.